RayandCarole Posted July 3, 2016 Share Posted July 3, 2016 I have sextant kicking around somewhere a relic from my seagoing days, goodness knows where it is tho Ray 1 Quote Link to comment Share on other sites More sharing options...
Vaughan Posted July 3, 2016 Share Posted July 3, 2016 This can also be done using Mils. These are used by artillery, naval gunnery and long range rifle shooting. One Mil is the angle subtended by one metre at 1000 metres. Get your GPS APP and position yourself 500 metres down (or up) the river from the bridge, then look at it through a telescopic rifle sight marked in Mil dots. If the angle at that range is just over 21 Mils, then the bridge height is 10.6 metres, or 35 feet. Quote Link to comment Share on other sites More sharing options...
Viking23 Posted July 3, 2016 Share Posted July 3, 2016 If you use water bags, they cannot sink, so once in the river will limit your maximum heel. You could get one of those flying camera things, then position it a few hundred yards behind the boat looking at the bridge, using parallax, you can see if you have enough clearance, but if all fails, save the video, and get £250 from "You've been framed" lol. 1 Quote Link to comment Share on other sites More sharing options...
Viking23 Posted July 4, 2016 Share Posted July 4, 2016 I was thinking about this, tie the water bag, or heavy weights to the boom, and push the boom out, solves the problem of getting the initial heel as well lol. Using the sine rule, if you heel 45 degrees, your mast height will have been shortened to around 71 % of it's height. Quote Link to comment Share on other sites More sharing options...
Vaughan Posted July 4, 2016 Share Posted July 4, 2016 1 hour ago, Viking23 said: Using the sine rule, if you heel 45 degrees, your mast height will have been shortened to around 71 % of it's height. I should think you will also fill the cockpit up with water, at that angle. There is also the thought that most if not all of the rudder will be out of the water, so no control. If (repeat if) I was going to do this, I would have a cruiser close abeam, and anchor the end of the jib halyard to it. You can then careen the yacht by hauling in on the halyard winch. good use of the speed of both boats' engines would keep everything straight as you go under. This would require seamanship of a high order, and I still wouldn't want to try it myself! 1 Quote Link to comment Share on other sites More sharing options...
Delta Posted July 12, 2016 Author Share Posted July 12, 2016 45 degrees was an illustration not a practical proposition and my rudder would not be out of the water the 71& is not 71% shorter it would be 71% of the height of the mast but no matter. Anyway, I now have the use of a laser measure now so I just need a lift to get me under the bridge or I may have to get my old slalom kayak out again... or deflate the dinghy and launch nearby but it does not have a licence these days so no go. Quote Link to comment Share on other sites More sharing options...
Viking23 Posted July 13, 2016 Share Posted July 13, 2016 The laser measure might not differentiate between the girders and the underside of the bridge, so be aware of that, so take many measurements to see if you can measure both for reference. Re boats, I would think someone might be around to help with this trial. Quote Link to comment Share on other sites More sharing options...
grendel Posted July 13, 2016 Share Posted July 13, 2016 with a laser measure you can position the red dot wherever you want, I have used them to measure the conductor height of overhead power lines, the only tricky bit there is getting the dot onto the wire for long enough, in that application anything over 10m becomes tricky if there is any wind. for measuring to a flat beam, it should have no problems. dont forget to add the height of the actual measure, plus the height between it and the water surface. mind you with the measure it should also be possible to get the height of the mast, if there is an almost horizontal surface to spot the dot onto at the right height. Quote Link to comment Share on other sites More sharing options...
Viking23 Posted July 13, 2016 Share Posted July 13, 2016 Some of the lower cost measuring devices, use a laser led to target, but an ultrasonic device to measure, ok for walls, ceilings etc the ultrasonic device is not pin point accurate, as it has a wide spread, and I doubt that many would be aligned with the laser led, so make sure you choose your technology carefully. Quote Link to comment Share on other sites More sharing options...
Delta Posted July 13, 2016 Author Share Posted July 13, 2016 Yes, thanks for that, it will/may be interesting to see what results I can get, as the laser beams form a circleof dots on the target and so some will fall on and some between any projections but should be esily visible around dusk. Realistically, I am only interested in how close to 35 or so feet it is, in the middle, so as to ascertain wehether or not there is any safety margin or whether the published height is actually given as the lowest point on the span which would be at the supports at each end. I have probably discounted attempting to travel with my masts fixed now but the question will not go away until I have had at least a good attempt at checking the height for myself. A single laser beam could be more accurate but I am not aware if there is a convenient source of such a calibrated item outside the military, scientific or surveying communities. It would not be likely to be affordable anyway. Quote Link to comment Share on other sites More sharing options...
grendel Posted July 13, 2016 Share Posted July 13, 2016 depending upon what you class as affordable, - http://www.laser-measure.co.uk/leica-disto/leica-disto-d2.html this is what I would call a decent laser measure - with 1.5mm accuracy. it was something similar to this that I trialed on overhead line heights, so at 10m it could pick up a 15mm wide target. This is very much bottom end of the survey laser measure market with only a 60m max range, though the range can be extended using retro reflectors (these reflect the beam at the same angle it comes in, so extend the capabilities of the measures - but do rely on you being able to place the reflector on the target. I suppose affordable depends upon how much a new mast could cost if you got it wrong? Quote Link to comment Share on other sites More sharing options...
grendel Posted July 13, 2016 Share Posted July 13, 2016 do note that some of the Leica laser measures have the option to get one on a free trial basis in the notes at the bottom of the information pages Quote Link to comment Share on other sites More sharing options...
TheQ Posted July 13, 2016 Share Posted July 13, 2016 Grendel are you offering to climb the bridge to fit the reflectors? Quote Link to comment Share on other sites More sharing options...
riyadhcrew Posted July 13, 2016 Share Posted July 13, 2016 Isn't it likely that the drawings that were submitted for planning permission would have all the relevant measurements on them? I'm sure that they must be available for someone to see. Please feel free to correct me if I'm wrong. Quote Link to comment Share on other sites More sharing options...
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